Hydatid disease, known medically as cystic echinococcosis, is caused by the larval stage of the tapeworm Echinococcus granulosus. The parasite cycles between dogs and livestock such as sheep, and humans become accidental hosts when they ingest tapeworm eggs shed in canine feces. Once inside the human body, the larvae travel through the bloodstream and typically settle in the liver, which accounts for 60 to 70 percent of cases, or the lungs, which account for 20 to 30 percent. The disease remains endemic across the Mediterranean basin, the Middle East, and parts of South America, where it poses a continuing public health challenge.

The spleen, however, is an extraordinarily rare destination for the parasite. Splenic involvement represents only 1.5 to 4 percent of all hydatid cases, and it usually occurs alongside disease in other organs. A truly isolated splenic cyst, with no trace of infection anywhere else in the body, is a clinical curiosity. The parasite is thought to reach the spleen either through the systemic circulation after slipping past the filtering capillary beds of the liver and lungs, or by traveling backward through the portal venous system. Because the organ is so rarely affected, clinicians seldom consider it early, and the slow-growing nature of the cyst compounds the problem, allowing the abdominal cavity to quietly accommodate the expanding mass for weeks or months.

That is precisely what happened in this patient. She reported no fever, no weight loss, no trauma, and no significant contact with dogs or livestock, though she lived in an endemic region. On examination, her abdomen was soft and non-tender, with no palpable mass and no enlargement of the spleen. Her complete blood count, liver function tests, and inflammatory markers were all within normal limits, and notably, she showed no hypereosinophilia, the elevation of a particular white blood cell type that often signals parasitic infection. By every conventional measure, she appeared entirely healthy.

The first clue came from imaging. An abdominal ultrasound revealed a finely echogenic cystic formation embedded within the splenic parenchyma. To characterize the lesion further, the team obtained a contrast-enhanced computed tomography scan, which showed a well-circumscribed, oval, hypodense, and homogeneous mass at the upper pole of the spleen, measuring 50 by 57 by 56 millimeters. The cyst wall was thin and regular, marked by a small area of peripheral calcification, and there was no internal septation, no endoluminal budding, and no contrast enhancement. These features were suggestive of a hydatid cyst, but they were not pathognomonic, meaning they could not definitively distinguish the parasite from other possibilities.

Then came the diagnostic twist. An enzyme-linked immunosorbent assay, or ELISA, specific for Echinococcus returned a negative result. The test was not repeated, and no complementary immunoblot or quantitative antibody measurement was available, so the surgical team could not rule out a low antibody titre or an assay-related false negative. The differential diagnosis remained broad: an epidermoid cyst of the spleen, a post-traumatic or post-infectious pseudocyst, and a cystic neoplasm all had to be weighed against the imaging pattern and the epidemiological context. The multidisciplinary team concluded that despite the negative serology, the radiological features in an endemic setting made a hydatid cyst the most probable diagnosis.

The biology behind the false negative is instructive. Serological testing for hydatid disease is highly sensitive for hepatic cysts, reaching 85 to 90 percent, but its performance drops significantly for splenic localizations. False negatives are frequently associated with intact cysts, dead or inactive parasites, or cysts with thick calcified walls that prevent antigen from leaking into the bloodstream. Without circulating antigen, the immune system never mounts the detectable antibody response that the test is designed to capture. In other words, the very features that made this cyst appear relatively benign on imaging, its intact wall and focal calcification, were the same features that allowed it to hide from the immune assay.

Faced with this uncertainty, the team chose an approach that balanced diagnostic confirmation with definitive treatment. Image-guided biopsy was rejected because of the risk of anaphylaxis and peritoneal seeding if a hydatid cyst were punctured. Instead, the patient underwent exploratory laparoscopy, with a plan to proceed to total splenectomy if intraoperative findings confirmed the diagnosis. Before surgery, she received vaccinations against Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type b, the encapsulated bacteria against which the spleen normally helps defend. No perioperative albendazole or other antihelminthic drug was given, because the diagnosis had not been definitively established before the operation.

The surgery itself demanded meticulous technique. Intraoperative exploration revealed a cystic mass at the upper pole of the spleen with an appearance highly consistent with hydatid disease. The team dissected under low pneumoperitoneum pressure, taking strict precautions to prevent cyst rupture and intraperitoneal spillage, including early isolation of the splenic hilum and avoidance of direct instrument contact with the cyst wall. Because of the cyst’s polar position and the risk of parasitic dissemination during partial resection, a total laparoscopic splenectomy was performed, and the specimen was extracted intact within an endoscopic retrieval bag. The postoperative course was uneventful, with no fever, bleeding, or signs of anaphylaxis, and the patient was discharged just two days after surgery. Histopathological examination of the removed spleen confirmed the diagnosis, revealing the characteristic acellular laminated membrane and germinal layer of Echinococcus granulosus.

At six-month follow-up, a repeat abdominal CT showed no evidence of recurrent or disseminated disease, and the patient reported feeling entirely well, having resumed her usual activities without limitation. The case carries several lessons for clinicians working in endemic regions. A cystic splenic mass should prompt consideration of hydatid disease regardless of serological results, because a negative test reflects the limitations of assay sensitivity rather than the absence of disease, particularly for calcified or extrahepatic cysts. Laparoscopic splenectomy with a retrieval bag is a safe and definitive treatment when strict anti-spillage precautions are respected, and vaccination against encapsulated organisms is essential whenever total splenectomy is anticipated, to mitigate the lifelong risk of overwhelming post-splenectomy infection. The authors caution that a single case report cannot be generalized, and longer follow-up beyond six months would strengthen confidence in the absence of recurrence, but the diagnostic pathway they document, from a negative blood test to a confirmed parasitic cyst, illustrates a pitfall that could easily catch an unsuspecting clinician anywhere the parasite still circulates.

Subject of Research: A rare isolated hydatid cyst of the spleen with negative serology diagnosed and treated by laparoscopic splenectomy

Article Title: Case Report: Isolated splenic hydatid cyst with negative serology managed by laparoscopic splenectomy

Article References: Safta, A. B., Nasri, S., Dougaz, W., Alouani, H., Ksentini, M., Jerraya, H., & Nouira, R. (2026). Case Report: Isolated splenic hydatid cyst with negative serology managed by laparoscopic splenectomy. Frontiers in Parasitology, 5, Article 1956525. https://doi.org/10.3389/fpara.2026.1956525

Image Credits: AI Generated

DOI: 10.3389/fpara.2026.1956525

Keywords: hydatid disease, Echinococcus granulosus, echinococcosis, splenic cyst, negative serology, laparoscopic splenectomy, parasitology, CT imaging, false-negative ELISA, zoonosis, spleen surgery, post-splenectomy vaccination